This invenion relates to signaling systems and, more particularly, to multifrequency signaling receivers.
Multifrequency signaling is now commonplace in communications systems. It is employed in subscriber signaling, signaling between central offices, interoffice communications, remote control of other systems, inputting data to computer systems, and the like. Consequently, it is increasingly important that detection of valid multifrequency signals be achieved accurately and inexpensively.
Heretofore, numerous arrangements have been proposed for detecting reception of "valid" multifrequency signals. A valid multifrequency signal is one which meets the established telecommunications objective of (a) two, and only two, tones being present for a prescribed interval, (b) one of the tones must have a magnitude greater than a first prescribed threshold level, for example greater than -5 dB relative to the single tone center of band output from the receiver automatic gain control (AGC) stage for the frequency of the particular multifrequency tone and, then, (c) another received tone is considered valid if it has a magnitude greater than a second prescribed threshold level, for example, greater than -10 dB relative to the single tone center of band output from the receiver AGC at the frequency of that tone. The -10 dB threshold corresponds to a tone having 6 dB of twist relative to the other received tone and being at the edge of the filter band for that particular tone.
Prior multifrequency receiver arrangements approximate the multifrequency reception objectives. In general, the prior known receiver arrangements are inefficient and somewhat cumbersome. They typically monitor two sets of circuit points simultaneously; one set of circuit points to determine if at least two tones have been received which exceed the second prescribed threshold of -10 dB, and the other set of circuit points to determine if the sum of the tones exceeds the first prescribed threshold of -5 dB. Since two separate sets of circuit points are being monitored simultaneously, additional hardware is required. Additionally, the monitoring arrangement for detecting whether the first threshold level has been exceeded typically employs a nonlinear summing arrangement which may be susceptible to noise signals and the like, thus yielding false indications of reception of "valid" multifrequency tones.